Galvanized steel is used in applications where rust resistance is needed, for instance, in the automotive industry where the zinc layer reduces the onset of rust at exposed edges and surfaces. Additional coating layers are provided to make the galvanised steel substrate more aesthetically pleasing and to further protect the steel substrate from corrosion. In this respect galvanised steel substrates are first provided with a phosphate coating (2-5 μm) to enhance paint adhesion, by enhancing paint adhesion such phosphate coatings indirectly enhance corrosion resistance.
The application of the phosphate coating comprises the steps of dissolving iron, zinc or manganese phosphate salts in a solution of phosphoric acid and immersing the steel substrate in the solution for 4-6 minutes. However, before the phosphate coating can be applied, the automotive manufacturer needs to clean the galvanised steel substrate, such that it is free from oil, grease, lubricants and rust. Cleaning the substrate and providing the phosphate layer is not trivial and increases manufacturing costs. The use of phosphoric acid also introduces environmental and safety issues related to the handling and disposal of such solutions.
Organic coatings are often provided on the phosphate coating by electrophoretic deposition, which is a process that uses electrical current to deposit paint onto a suitable substrate. The electro-(organic)-coating typically has a thickness between 7 and 20 μm and acts as a primer for the application of additional paint layers. The use of electrophoretic deposition to apply organic coatings increases the cost of manufacturing automotive parts due to the high voltages that are required to apply such coatings.
It is an object of the present invention to provide a coated steel substrate having improved corrosion resistance, adhesion properties and lubrication properties.
It is a further object of the invention to provide a coated steel substrate having a reduced coating thickness.
It is another object of the invention to provide a process for manufacturing a coated steel substrate in which the steps of providing a phosphate coating and an electro-coating are avoided.